1. Field of the Invention
This invention relates generally to electronic circuits for the controlled switching of high current loads and more particularly relates to controlling a solid state, smart, high side, high current, driver, that has internal current and temperature sensing, in a manner that it is enabled to switch incandescent lamps ON within a practical time interval.
2. Description of the Related Art
Convenient, energy efficient operation of electrical equipment, appliances and other electrical devices, such as flashers for periodically illuminating signaling or warning lights, often requires a switch for turning the device on and off. This is particularly important when multiple different devices are powered by a vehicle battery in order to minimize drain on the battery by permitting selected operation of only those devices that are currently being used. Some electrical devices are high power devices that draw large currents. For those, it is often desirable to control the switch that switches the high current from a low power electrical command signal. For this purpose, relays or power contactors were traditionally used. However, such devices have mechanical electrical contacts which are subject to corrosion and the possibility of having the contacts welded together and suffer from numerous other failure modes.
When solid state technology appeared, it was first applied to develop solid state switches that could be used for switching low currents associated with low power applications but the available solid state switching devices could not tolerate the higher currents of higher power loads. However, MOS/FET devices have more recently been developed that can switch currents on the order of a hundred amperes or more. These switching devices have thousands of MOS/FETs formed in an integrated circuit and connected in parallel so they each carry a small portion of the current and operate together as a high current, composite MOS/FET.
Additionally, modern integrated circuit technology also permits a variety of other circuits to be formed in the same integrated circuit to provide for operating, controlling and protecting the MOS/FETs. Because these associated circuits include digital logic and microcontrollers and include sensing circuits that can detect a variety of fault conditions and, in response to sensed conditions programmed into the digital control circuits, turn off the MOS/FETs in order to protect the integrated circuit module, the modules are called “smart”.
Consequently, the combination of the current switching MOS/FETs and their associated circuits provides high power switch modules that manufacturers can use to construct high power switches for applications in their field. In addition to the relatively high currents that each integrated circuit module can switch, manufacturers of high power switches can connect multiples of these integrated circuit modules in parallel to increase the maximum current their products can switch by a multiplier equal to the number of parallel modules. The use of these switching devices is increasing because there is no price differential between the older conventional contactors and the newer, smart, MOS/FET switches. A designer chooses the switches with the lowest drain to source resistance, RDS ON or RDS, because they give the lowest heat dissipation in the MOS/FETs.
A particularly useful type of power switching module is a smart, high side, high current driver. High side means that the power switching terminals are connected between the high side of the power source, such as the positive terminal of a battery, and the load being switched. The second terminal of the load is connected to a common power circuit ground. Such drivers also provide a terminal connected to an additional MOS/FET source which is part of a current mirror circuit that provides a current signal to an external circuit that is proportional to the load current. There are a few manufacturers who supply such modules such as a PROFET® BTS555 offered by Infineon Technologies. This first generation high current power switch module was designed to control a starting motor on a vehicle.
The next step in the evolution of the high power MOS/FET high side drivers was current limiting. An additional MOS/FET source is used for an additional current minor to provide a sensed current signal to the internal control circuitry of the high side driver that is proportional to the load current. Such high side drivers have a digital logic control input that can be controlled by external logic circuitry, such as a microcontroller, which commands the driver to an ON state or an OFF state. The internal control circuits and the high current switching MOS/FETs can be on one integrated circuit or, more commonly, on multiple integrated circuits that are wired together. The high side driver is permanently grounded and power is permanently applied to it to power its control circuitry. Consequently, the internal control circuit of such high side drivers is able to continuously sense the output load current and modulate the load current by varying the gate voltage including switching OFF the load current by clamping the gate to zero in response to detected fault conditions.
In addition to current sensing and current limiting, the second generation high side drivers also have temperature sensors that have an output connected to their internal control circuit for sensing the temperature of the MOS/FETs and controlling the load current. Consequently, their control circuits are able to modulate or turn OFF the load current as a function of this temperature and thereby also protect the high side driver from excessive temperatures.
An example of a prior art high side driver is a VND5004-E sold by STMicroelectronics. This driver has a minimum 4 milliohms RDS ON and its circuit is illustrated in FIG. 2. The VND5004-E driver has two current switching circuits for independently controlling two loads, although for some applications the two are advantageously connected in parallel to distribute the current between them and allow twice the maximum current of each. Each switching circuit includes two composite MOS/FETs 10 and 12 and several associated control circuits for operating, controlling and protecting the MOS/FETs. The control circuits include a logic circuit 14, such as a microcontroller or other digital logic circuit, programmed with suitable instructions for performing the control processes. The operating, controlling and protecting circuits include circuits for sensing load current and MOS/FET temperature as well as other circuits but they are not described in detail because they are a part of the prior art and are described in the manufacturer's specifications.
High side drivers of this type also have terminals for connecting the driver to external circuitry. The VND5004-E driver has outputs 16 and 18, one for each of its two switching circuits, for connection to separate loads or the same load. It also has control inputs 20 and 22 for respectively controlling each of the high current, composite MOS/FETs 10 and 12. The other terminal of a load is connected to the power circuit ground. The power source, such as a 12 or 24 volt vehicle battery, is connected to a battery terminal 24 and the other power source terminal 26 is connected to the power circuit ground. The VND5004-E driver has a pair of sensing or diagnostic terminals 28 and 30 which are outputs of internal current minor circuits. Each diagnostic terminal provides an output current that is proportional to the load current through its associated composite MOS/FET 10 and 12 respectively. Another current minor circuit for each MOS/FET is used for sensing the same output load currents but provides its output signal to the internal logic of the high side driver. Each current mirror circuit utilizes a few of the large number of individual MOS/FETs formed in the integrated circuit of the high side driver. A current minor circuit is well known in the art and, as applied to the drivers, the drains of all the individual MOS/FETs are connected together and most of the sources of the individual MOS/FETs are connected to the main output load terminal 16 or 18 for switching the high load current. A few of the sources, however, are instead used for the current minor circuits and connected to the sensing terminals 28 and 30 or to provide the internal feedback load current signal. As known in the art, a current mirror circuit provides a current that is proportional to the load current through the majority of MOS/FETs that conduct the load current through the driver.
The operation of the VND5004-E driver provides an example of the operation of the second generation drivers. The internal logic control circuits provide generally the following protective functions. In the event that the internal current sensor detects an overload current that exceeds a selected maximum current limit ILIMH, such as 100 amps for example, the internal control logic decreases the voltage applied to the MOS/FET gates to maintain the current at the maximum limit. Unfortunately, this increases the drain to source resistance RDS which results in more heat dissipation in the MOS/FETs. Because the current equals the applied source voltage divided by the sum of the load resistance and the drain-source resistance RDS, in order to limit the current to a fixed current limit, the less the resistance of the load, the greater must be the drain-source resistance RIDS. Therefore, when the driver is operating at a particular current limit, the less the resistance of the load, the greater the heat dissipation in the MOS/FETs.
Because the internal control circuit of the driver also senses MOS/FET temperature, in the event that the sensed temperature exceeds a maximum temperature TTSD, the internal control logic shorts the MOS/FET gate to ground and thereby opens the MOS/FETs to a non-conductive, high resistance state. Consequently, if a MOS/FET gets too hot, the high side driver is shut down by its own associated, internal control circuitry before the driver is damaged.
Additionally, the second generation drivers are programmed to turn the MOS/FETs back on after their temperature drops to an intermediate temperature TR as a result of heat transfer by conduction from the MOS/FETs through heat sinks and into the surrounding environment. However, under the described conditions, the MOS/FETs are turned back on at a reduced maximum current limit ILIML, such as 40 amps, for example. Thereafter, if the MOS/FET temperature again rises to the maximum temperature TTSD, the MOS/FETs are again shut down to a non-conductive, high resistance state to stop all current flow. Then, when the temperature again falls below the intermediate temperature TR the MOS/FETs are again turned on but again at the reduced maximum current limit ILIML. This operation then repeats unless and until the temperature sensors detect that the MOS/FETs have cooled to a lower temperature TRS. This reduced current limit is imposed by reducing the MOS/FET gate voltage which, as previously mentioned, results in an increased drain to source resistance RDS and more dissipation of electrical energy as heat in the MOS/FET. This increased drain to source resistance RDS is greater than the specified minimum drain to source resistance, such as the 0.004 ohms described above, and therefore dissipates more heat in the MOS/FETs than would an RDS equal to the minimum RDS.
There is an important reason that the high side driver is programmed to limit the current to a lower current limit ILIML following the detection of an excessive temperature above the shut down temperature TTSD. In order to optimize the usefulness of the driver, it is desirable to have the broadest range of normal operating conditions; i.e. the highest current limit and the highest permissible temperature of operation. However, silicon is brittle and if repeatedly cycled between wide temperature extremes, it will eventually be damaged (e.g. crystal fracture) and become inoperable. The larger the temperature differential over which the device cycles, the shorter the lifetime of the device. In order to maximize the current limitation, the internal control circuit of the driver initially turns the driver ON at its maximum current limit ILIMH. The internal control circuit turns it OFF if the temperature reaches the maximum temperature TTSD. However, the driver control circuit thereafter allows it to cool to an intermediate temperature TR but then limits its load current to a lower current limit ILIML in order to protect the driver. Consequently, the temperature algorithm used by the internal control circuit has multiple temperature levels so the MOS/FETs will cycle between two, not so extreme temperatures after its temperature has exceeded its maximum temperature TTSD. So, in summary, in order to get the broadest range of operating characteristics, the device first applies its highest current limit ILIMH, such as the 100 amp current limit of the VND5004A-E, which, under most and/or normal conditions will not be exceeded or cause an overtemperature. However, once the control circuit of the driver turns the driver OFF as a result of an excessive temperature, it then goes to a much lower current limit and turns the driver back ON when the MOS/FET temperature falls to an intermediate level in order to prevent cycling between excessive temperature extremes so as to reduce the mechanical stress on the silicon device.
If the driver is controlling the current through any fixed impedance load, the above-described operation works fine. If a dead short is applied to the load terminals, the internal control circuit switches the load OFF and also protects driver because there are no excessive currents.
However, there is a problem when a high side driver having this control algorithm is applied to controlling the current through incandescent lamps, such as, for example, the incandescent lamps on a motor vehicle. Some motor vehicles have large numbers of incandescent lamps, particularly emergency vehicles, such as ambulances, emergency medical service vehicles or fire trucks. Not only do they often have a large number of lamps, but they also have high power, high light intensity lamps. Such emergency motor vehicles typically are equipped with many exterior lamps for signaling the presence of the vehicle, interior lamps for lighting their interior work areas and lamps for illuminating areas outside and around the vehicle. One example of a switching module that advantageously uses a smart, high side driver is a flasher for the exterior signal lights of an emergency motor vehicle. Its function is to turn the lights ON and OFF in a periodic manner to increase the visibility of the vehicle and heighten the awareness of its presence and movement by nearby motorists. The problem is that the second generation, smart, high side drivers of the type described are unable to turn on many of the incandescent lamp loads presented by such vehicles within a useful time interval. For example, for some banks of lamps, the incandescent lamps may take on the order of a minute to be turned on by the driver. For some lamps with an even higher power demand, the lamps can never be turned on by the high side driver using the above-described control algorithm.
It is therefore an object and feature of the invention to provide a circuit and a method of operating smart, high side drivers of the type described so that the drivers are able to turn on loads that include incandescent lamps (or other electrical loads that have a similar characteristic) that interact with the high side driver to prevent the driver from turning on the load within an acceptable and practical time interval.